Congenital abnormalities of the upper (kidney) and lower (ureter, bladder, urethra and external genitalia) urinary tracts are diagnosed at a rate of 1 in 500 prenatal ultrasounds. These conditions contribute to significant chronic morbidity in affected children, necessitating multiple surgeries and life-long treatment. Currently, our understanding of the underlying cause(s) of these disorders is limited. My mentor, Dr. Xue (Sean) Li, has shown that the Six1-Eya1 transcriptional complex is indispensable to kidney development because a mutation in either Six1 or Eya1 causes renal agenesis in the mouse. I have subsequently demonstrated that the Six1-Eya1 transcriptional complex is also required for lower urinary tract formation. In the preliminary studies, I found that mouse embryos without Six1 and Eya1 genes display morphological defects similar to the human persistent cloaca syndrome. This syndrome is characterized by a failure of the lower urinary and intestinal/rectal tracts to separate during embryonic development. To understand the cellular and molecular origins of these defects and to begin to shed light onto the molecular mechanisms of mammalian lower urinary system development, I propose the following specific aims: Aim 1, To characterize the histological and molecular origins of the persistent cloaca phenotype seen in the Six1(-/-)::Eya1(-/-) double null mutant embryos. This aim will test the hypothesis that the Six1-Eya1 transcriptional complex regulates expression of target genes that are required for the morphogenesis of the genitourinary structures. Aim 2, To pinpoint the cellular origins of the lower urinary tract using a Cre/loxP site-specific DNA recombination fate mapping strategy. This aim will test the hypothesis that Six1-expressing cells contribute directly to lower genitourinary tract tissues. Aim 3, To determine if the TGF(-Smad4 signaling pathway is required in the process of lower genitourinary tract development. This aim will test the hypothesis that the TGF(-Smad4 signaling pathway is essential for development of the urogenital structures. Along with these scientific goals, my mentors and I have devised a detailed strategy for in-depth development of my academic skills during the K08 period. This training program will be conducted at Children's Hospital Boston, an outstanding setting for biomedical research and career development.